i) Field of the Invention
This invention relates to a composition for forming a repulpable, water resistant, grease resistant, water vapor permeable coating on a porous paper sheet material; a repulpable coated paperboard liner for use in corrugated paperboard packaging manufacture, a method of producing a repulpable coated paperboard liner; a repulpable corrugated paperboard and a method of producing a repulpable corrugated paperboard.
ii) Description of Prior Art
Perishable food products such as frozen meat, fish and vegetables are shipped, stored and displayed for sale in packages formed of corrugated paperboard coated with a water resistant or repellant material, the most widely used coating being a wax coating. The wax coating renders the packages resistant or repellant to water and impermeable to water vapor.
Such packages, however, have a serious disadvantage in that they are essentially non-repulpable. Repulping refers to the art of recycling waste paper products including paperboard to provide a reusable wood pulp. Recyclability of waste paper products is of growing importance in efficiently employing available wood fibre resources and in curtailing the amount of waste which is either combusted or disposed of in landfill sites.
The problem in recycling wax-coated paperboard is well established and is described in, for example, U.S. Pat. Nos. 5,626,945 and 5,562,980 and PCT International Publication WO 96/22329.
An additional problem or manufacturing limitation with existing water repellent coatings, such as wax coatings, for corrugated paperboard packaging, is that the wax coating can only be applied to the finished or assembled corrugated paperboard.
Corrugated paperboard comprises an assembly of a pair of spaced apart linerboards with a corrugated medium sandwiched therebetween and adhered thereto. In particular, the tips of the flutes of the corrugated medium are adhered by an adhesive to the inwardly facing surfaces of the linerboards. The most widely used adhesives are aqueous starch-based adhesives which meet Government standards, e.g., FDA standards, for food packages. After assembly the adhesive is dried liberating water vapor which escapes through the porous uncoated linerboards.
If the wax coating were to be applied to the linerboards prior to assembly of the corrugated paperboard, the vapor impermeable wax coating would prevent the escape of the water vapor from the adhesive and the water vapor would be trapped and condense in the interior of the corrugated paperboard.
Consequently employing conventional water vapor impermeable coatings such as the wax coatings, necessitates complete assembly and formation of the corrugated paperboard before application of the vapor impermeable coating. This places significant restrictions on the manufacturing process.
It is an object of this invention to provide a composition for forming a water-resistant, grease resistant, water vapor permeable coating on a porous paper sheet material.
It is a further object of this invention to provide such a composition having the quality that the thus coated porous paper sheet material is repulpable.
It is a further object of this invention to provide such a composition which forms such a coating which is also heat resistant.
It is still another object of this invention to provide a repulpable coated paper substrate for packaging, especially a coated paperboard liner for use in corrugated paperboard packaging manufacture.
It is yet another object of this invention to provide a method of producing the afore-mentioned repulpable coated paper substrate for packaging, especially a coated paperboard liner.
It is a further object of this invention to provide a repulpable corrugated paperboard.
It is a still further object of this invention to provide a method of producing the afore-mentioned repulpable corrugated paperboard.
In accordance with one aspect of the invention there is provided a composition for forming a water resistant, grease resistant, water vapor permeable coating on a porous paper sheet material, comprising: i) a styrene-acrylate copolymer; ii) a C14-C18 fatty acid complex of a metal ion having an oxidation state of at least 3, and iii) an aqueous vehicle.
In accordance with another aspect of the invention there is provided a repulpable coated paperboard liner for use in corrugated paperboard packaging manufacture comprising: a) a paperboard sheet having opposed first and second surfaces, b) a water resistant, grease resistant, water vapor permeable coating on at least one of said surfaces, said coating comprising a styrene-acrylate copolymer and a C14-C18 fatty acid complex of a metal ion having an oxidation state of at least 3.
In accordance with still another aspect of the invention there is provided a method of producing a repulpable coated paperboard liner for use in corrugated paperboard packaging manufacture comprising: a) providing a paperboard sheet having opposed first and second surfaces, b) coating at least one of said surfaces with a composition comprising: i) styrene-acrylate copolymer, ii) a C14-C18 fatty acid complex of a metal ion having an oxidation state of at least 3, and iii) an aqueous vehicle, and c) drying said composition to form a repulpable, water resistant, grease resistant, water vapor permeable coated linerboard.
In accordance with yet another aspect of the invention there is provided a repulpable corrugated paperboard comprising: a) first and second coated paperboard liners and a corrugated medium therebetween, each of said liners having opposed inner and outer surfaces, and said corrugated medium being adhesively secured to said inner surfaces, b) at least one of said outer surfaces having a water resistant, grease resistant, water vapor permeable coating thereon, and c) said corrugated medium being water vapor impermeable.
In accordance with a further aspect of the invention there is provided a method of producing a repulpable corrugated paperboard comprising: a) providing paperboard sheet having opposed first and second surfaces, b) coating at least one of said surfaces with a coating composition which dries to form a water resistant, grease resistant, water vapor permeable coating, c) drying said composition to form a repulpable, water resistant, grease resistant, water vapor permeable coated paperboard liner, d) adhesively securing a water vapor impermeable corrugating medium between a pair of paperboard liners, at least one of said liners being a said coated paperboard liner, with a water-based adhesive, and e) curing said adhesive, allowing water vapor of said water-based adhesive to escape through said at least one liner.
i) Descriptions
Certain terms are employed herein to identify characteristics of the coating composition, the coated paperboard liner and the corrugated paperboard. These terms are believed to be clear in the context of the invention, to a person skilled in the art but are further elaborated here.
The terms xe2x80x9cwater repellentxe2x80x9d and xe2x80x9cwater resistantxe2x80x9d refer to the tendency of the coating to repel, block or, in any event, not transmit or absorb any significant quantity of liquid water in normal use. In other words, these terms identify a liquid water-blocking property of the coating sufficient for packing intended for perishable frozen foods.
The term xe2x80x9cgrease resistantxe2x80x9d refers to the character of the coating in repelling, blocking or, in any event, not transmitting or absorbing any significant quantity of grease or oil.
The term xe2x80x9cwater vapor permeablexe2x80x9d refers to the character of the coating in permitting passage therethrough of water in vapor form.
The term xe2x80x9cwater vapor impermeablexe2x80x9d refers to the character of the corrugating medium in not permitting passage therethrough of water in vapor form.
The term xe2x80x9crepulpablexe2x80x9d refers to the character of the coated paperboard liner and the corrugated paperboard, whereby the paper fibre component of the paperboard liner and the corrugated paperboard can be readily recovered as a pulp suitable for use in paper product manufacture, the coating composition of the invention not presenting any significant obstacle to such pulp recovery. The term is to be viewed in the context that the conventional wax coated paperboard liners employed in corrugated paperboard packages for perishable frozen foods, are considered essentially non-repulpable based on a number of factors including the difficulty in separating the wax coated paper fibres and the contamination of any fibre pulp produced with wax particles which form stickies rendering the pulp unsuitable for paper manufacture.
As such the term xe2x80x9crepulpablexe2x80x9d contemplates absence or substantial absence of wax or comparable materials which would render the paperboard liner and corrugated paperboard non-repulpable.
The term xe2x80x9cheat resistantxe2x80x9d refers to the character of the coating in not becoming soft or tacky at the manufacturing temperatures encountered in the manufacture of corrugated paperboard, more especially at temperatures below about 200xc2x0 C.
In the present invention the corrugated paperboard has the necessary characteristics for packaging of perishable frozen foods, including water-repellency, grease resistance and water vapor impermeability. The water repellency and grease resistance, but not the water vapor impermeability, are provided by the coated paperboard liners; the water vapor impermeability is provided by the corrugating medium.
The employment of the coating composition of the invention which provides a coating for the porous paperboard liners, which is water vapor permeable, permits considerable variation in the manufacture of the corrugated paperboard.
Thus the corrugated paperboard may be assembled employing pre-coated paperboard liners which are water vapor permeable so that post-coating of the manufactured corrugated paperboard is not required, thereby considerably simplifying the manufacturing process of the corrugated paperboard.
ii) Coating Composition
The coating composition of the invention forms a water resistant, grease resistant, water vapor permeable coating on a porous paper sheet material, such as paper linerboard employed in the manufacture of corrugated paperboard.
The coating composition has particular application for the coating of the paper linerboard of corrugated paperboard employed in packaging in which the paperboard is required to be water resistant or repellent and grease resistant, such as in packaging for perishable frozen foods.
a) Copolymer
The coating composition comprises a styrene-acrylate copolymer and a C14-C18 fatty acid complex of a metal ion having an oxidation state of at least 3, in an aqueous vehicle.
The copolymer is, in particular, a copolymer of styrene and an alkyl acrylate in which the alkyl moiety has 1 to 6 carbon atoms. Butyl acrylate is especially preferred as the comonomer of styrene.
One particular commercially available source of the copolymer is the styrene/butyl acrylate copolymer dispersion available under the Trade-mark ACRONAL 296 D from BASF and which is described as a dispersion in water having a content of the copolymer of about 50%, (50%xc2x11), a pH of 7.5 to 9, a viscosity at 23xc2x0 C. (ISO 3219) of 300 to 650 mPas, and an apparent Brookfield viscosity at 23xc2x0 C. of about 10,000 mPas.
The dispersion is further described as being anionic, having a density of about 1.04 g/cm3, and having an average particle size of particles of the copolymer of about 0.1 xcexcm.
The dispersions of the styrene-acrylic copolymer may typically contain 40 to 60%, preferably about 50%, by weight of the copolymer.
b) Complex
The complex of the metal ion is in particular a complex of a metal having an oxidation state of at least 3, for example, an ion of iron, titanium, chromium or vanadium. The C14-C18 fatty acid of the complex may be, for example, tetradecanoic acid also known as myristic acid; or octadecanoic acid also known as stearic acid. The complex may comprise a mixture of C14-C18 fatty acids.
A commercially available class of complexes suitable in the invention is that available under the Trade-mark QUILON of DuPont Company which class comprises solutions of a chemically reactive complex in which a C14-C18 fatty acid is coordinated with trivalent chromium. The vehicle of the solutions is primarily an alkanol, usually isopropanol.
Particular complexes include chromium pentahydroxy (tetradecanoato)di- available in solution under the Trade-marks QUILON C and C-9 and which are catalogued by CAS Registry No. 65229-24-5; tetradecanoato chromic chloride hydroxide available in solution under the Trade-mark QUILON M and catalogued by CAS Registry No. 15659-56-0; and octadecanoato chromic chloride hydroxide available under the Trade-mark QUILON S and catalogued by CAS Registry No. 15242-96-3.
These complexes are considered to have a structure of the following form 
in which R is a fatty acid radical of 13 to 17 carbon atoms and Rxe2x80x2 is the alkyl group of the alkanol vehicle, for example, isopropanol.
When the alcoholic solution of the complex is diluted with water, aquo groups replace the coordinated alcohol groups and some of the chlorine atoms. The chlorine atoms enter solution as chloride ions and the complex acquires a positive charge; the complexes may polymerize through hydroxyl bridges, as a result of hydrolysis or neutralization.
The different grades of QUILON are described by DuPont Company as having the following typical properties set forth in Table I
Especially good results were achieved with QUILON C which also provides coating compositions displaying good storage stability.
The complex of the invention is more especially a so-called Werner complex.
The alcoholic solution of the complex may suitably contain about 3 to about 15%, preferably about 4 to about 12%, more preferably about 5 to about 10%, by weight of the metal ion, especially chromium and about 8 to about 28%, preferably about 10 to about 25%, by weight of fatty acid; and may typically contain chloride in an amount of about 5 to about 20%, preferably about 7 to about 15%, by weight.
c) Other Components
The aqueous vehicle of the coating composition comprises the water of the aqueous dispersion of the copolymer and the alkanol of the solution of the complex.
The coating composition may additionally comprise an inert particulate filler, for example, clay and may contain water in addition to that derived from the aqueous dispersion of the copolymer, which in this Specification is referred to as xe2x80x9cadditional waterxe2x80x9d. The additional water is employed to lower the viscosity, if desired.
In general, the coating composition may contain in weight % to a total of 100%:
Amounts of the alcoholic solution above 5% result in loss of stability and shorten the shelf life of the coating composition.
The particulate filler which is suitably a clay filler such as that available under the Trade-mark Omnifil provides heat resistance in the coating.
Such heat resistance is advantageous to the extent that heat is employed in the manufacture of the corrugated paperboard, for example, in the adhesion of the coated liner board to the corrugated medium, where the coating on the coated linerboard may be in direct contact with a heated pressure roll or platens which pressure roll or platens may typically be at a temperature of about 177xc2x0 C.
Preferably the filler is present in an amount of 10 to 15%, by weight. Amounts of the filler above 35%, by weight, result in deterioration in the water and grease resistance.
The coating composition applied to porous paper sheet material, such as linerboard for use in corrugated paperboard manufacture, forms a water resistant, grease resistant coating which is permeable to water vapor, and the coated paper sheet material is pulpable, the coating separating readily from the paper sheet material during repulping.
The coating composition is applied in a dry coating weight of 5 g/m2 to 25 g/m2. Typically the lower coating weights will be employed for smooth surface paper substrates and higher coating weights for more porous surface paper substrates.
It will be understood that the coating composition of the invention is a wax-free composition, although small amounts of wax might be tolerated to the extent that the coated linerboard still retains its repulpability and the coating is water vapor permeable.
iii) Water Impermeable Coating
The coating composition of the invention is particularly employed for coating linerboard for use in corrugated paperboard manufacture.
In such case the corrugated medium of the invention is coated to produce a water vapor impermeable barrier to provide the required water vapor impermeability in the corrugated paperboard.
The coating employed should be one which renders the corrugated medium repulpable so that the repulpable coated corrugated medium in conjunction with the repulpable coated linerboard is repulpable.
The coating providing water-vapor impermeability may be, for example, the coating described in WO 96/22329, published Jul. 25, 1996, S. Berube, the teachings of which are incorporated herein by reference. In general the afore-mentioned publication describes a moisture vapor barrier coating for a paper substrate, which produces a repulpable coated paper substrate, the moisture vapor barrier coating is more especially based on polyvinylidene chloride, although other polymer materials are also described. The polymer is employed in emulsion from and may additionally contain an additive such as hydrated aluminum silicate, calcium carbonate or vinyl acetate homopolymer.
Another preferred moisture vapor barrier containing composition for forming the water-vapor impermeable coating comprises a combination of polyvinylidene chloride, styrene-butadiene copolymer and an acrylic polymer. This composition is employed as a dispersion of the three polymers in an aqueous vehicle and may be applied between a pair of paper substrates which are then laminated together, the composition forming a water-vapor impermeable coating or layer therebetween. This laminate is then corrugated to form the corrugated medium.
Alternatively the composition may be applied to one or both sides of a paper substrate to form a coated substrate which is then corrugated to form the corrugated medium. It is also possible to apply the composition as a coating to a preformed corrugated medium.
The water impermeable coating is wax-free, although small amounts of wax might be tolerated to the extent that the resulting water impermeable corrugated medium retains its repulpability.